Added moar ContentCapture APIs (and their initial implementation).

There are 4 new APIs on View:
  - boolean setImportantForContentCapture()
  - boolean getImportantForContentCapture()
  - boolean isImportantForContentCapture()
  - boolean onProvideContentCaptureStructure()

And 4 on IntelligenceManager:
  - void notifyViewAppeared()
  - void notifyViewDisappeared()
  - void notifyViewTextChanged()
  - ViewStructure newVirtualViewStructure()

These methods are similar to the equivalent methods that are used for Autofill
and/or Assist, except for the following differences:

- The view hierarchy nodes are reported as they are rendered, rather than at
  once in a tree, recursively. Hence, the ViewStructure implementation does
  not implement the methods that add children to it, and views that provide
  virtual hierarchies must manually call IntelligenceManager to create the
  ViewStructure to their children and notify when their children are added and
  removed.
- It does not support methods added for Autofill to handle HTML pages (such
  as setHtmlInfo() and setWewbDomain()), as they're not important in the
  Content Capture context.
- Similarly, it also does not support setDataIsSensitive(), because the
  Intelligence service does not have the same restrictions as the Autofill
  service.

The CL also provides the initial implementation of these APIs, although still
full of TODOs (for example, we're not holding the events to send as a batch
yet).

Test: m -j update-api doc-comment-check-docs
Bug: 117944706

Change-Id: I43f06ce82bfe3b14d8d13fb3b2ebee223db83284